HTTPbis Working Group R. Fielding, Ed.
Internet-Draft Adobe
Obsoletes: 2616 (if approved) J. Gettys
Intended status: Standards Track Alcatel-Lucent
Expires: September 15, 2011 J. Mogul
HP
H. Frystyk
Microsoft
L. Masinter
Adobe
P. Leach
Microsoft
T. Berners-Lee
W3C/MIT
Y. Lafon, Ed.
W3C
M. Nottingham, Ed.
J. Reschke, Ed.
greenbytes
March 14, 2011
HTTP/1.1, part 6: Cachingdraft-ietf-httpbis-p6-cache-13
Abstract
The Hypertext Transfer Protocol (HTTP) is an application-level
protocol for distributed, collaborative, hypermedia information
systems. This document is Part 6 of the seven-part specification
that defines the protocol referred to as "HTTP/1.1" and, taken
together, obsoletes RFC 2616. Part 6 defines requirements on HTTP
caches and the associated header fields that control cache behavior
or indicate cacheable response messages.
Editorial Note (To be removed by RFC Editor)
Discussion of this draft should take place on the HTTPBIS working
group mailing list (ietf-http-wg@w3.org). The current issues list is
at <http://tools.ietf.org/wg/httpbis/trac/report/3> and related
documents (including fancy diffs) can be found at
<http://tools.ietf.org/wg/httpbis/>.
The changes in this draft are summarized in Appendix C.14.
Status of This Memo
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Internet-Draft HTTP/1.1, Part 6 March 2011
This Internet-Draft is submitted in full conformance with the
provisions of BCP 78 and BCP 79.
Internet-Drafts are working documents of the Internet Engineering
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Internet-Drafts are draft documents valid for a maximum of six months
and may be updated, replaced, or obsoleted by other documents at any
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material or to cite them other than as "work in progress."
This Internet-Draft will expire on September 15, 2011.
Copyright Notice
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Contributions published or made publicly available before November
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Table of Contents
1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . 51.1. Purpose . . . . . . . . . . . . . . . . . . . . . . . . . 51.2. Terminology . . . . . . . . . . . . . . . . . . . . . . . 51.3. Requirements . . . . . . . . . . . . . . . . . . . . . . . 7Fielding, et al. Expires September 15, 2011 [Page 2]

Internet-Draft HTTP/1.1, Part 6 March 20111. Introduction
HTTP is typically used for distributed information systems, where
performance can be improved by the use of response caches. This
document defines aspects of HTTP/1.1 related to caching and reusing
response messages.
1.1. Purpose
An HTTP cache is a local store of response messages and the subsystem
that controls its message storage, retrieval, and deletion. A cache
stores cacheable responses in order to reduce the response time and
network bandwidth consumption on future, equivalent requests. Any
client or server MAY employ a cache, though a cache cannot be used by
a server that is acting as a tunnel.
Caching would be useless if it did not significantly improve
performance. The goal of caching in HTTP/1.1 is to reuse a prior
response message to satisfy a current request. In some cases, a
stored response can be reused without the need for a network request,
reducing latency and network round-trips; a "freshness" mechanism is
used for this purpose (see Section 2.3). Even when a new request is
required, it is often possible to reuse all or parts of the payload
of a prior response to satisfy the request, thereby reducing network
bandwidth usage; a "validation" mechanism is used for this purpose
(see Section 2.4).
1.2. Terminology
This specification uses a number of terms to refer to the roles
played by participants in, and objects of, HTTP caching.
cache
A conformant implementation of a HTTP cache. Note that this
implies an HTTP/1.1 cache; this specification does not define
conformance for HTTP/1.0 caches.
shared cache
A cache that is accessible to more than one user; usually (but not
always) deployed as part of an intermediary.
private cache
A cache that is dedicated to a single user.
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Internet-Draft HTTP/1.1, Part 6 March 2011
cacheable
A response is cacheable if a cache is allowed to store a copy of
the response message for use in answering subsequent requests.
Even when a response is cacheable, there might be additional
constraints on whether a cache can use the stored copy to satisfy
a particular request.
explicit expiration time
The time at which the origin server intends that a representation
no longer be returned by a cache without further validation.
heuristic expiration time
An expiration time assigned by a cache when no explicit expiration
time is available.
age
The age of a response is the time since it was sent by, or
successfully validated with, the origin server.
first-hand
A response is first-hand if the freshness model is not in use;
i.e., its age is 0.
freshness lifetime
The length of time between the generation of a response and its
expiration time.
fresh
A response is fresh if its age has not yet exceeded its freshness
lifetime.
stale
A response is stale if its age has passed its freshness lifetime
(either explicit or heuristic).
validator
A protocol element (e.g., an entity-tag or a Last-Modified time)
that is used to find out whether a stored response is an
equivalent copy of a representation.
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Internet-Draft HTTP/1.1, Part 6 March 20111.3. Requirements
The key words "MUST", "MUST NOT", "REQUIRED", "SHALL", "SHALL NOT",
"SHOULD", "SHOULD NOT", "RECOMMENDED", "MAY", and "OPTIONAL" in this
document are to be interpreted as described in [RFC2119].
An implementation is not compliant if it fails to satisfy one or more
of the "MUST" or "REQUIRED" level requirements for the protocols it
implements. An implementation that satisfies all the "MUST" or
"REQUIRED" level and all the "SHOULD" level requirements for its
protocols is said to be "unconditionally compliant"; one that
satisfies all the "MUST" level requirements but not all the "SHOULD"
level requirements for its protocols is said to be "conditionally
compliant".
1.4. Syntax Notation
This specification uses the ABNF syntax defined in Section 1.2 of
[Part1] (which extends the syntax defined in [RFC5234] with a list
rule). Appendix B shows the collected ABNF, with the list rule
expanded.
The following core rules are included by reference, as defined in
[RFC5234], Appendix B.1: ALPHA (letters), CR (carriage return), CRLF
(CR LF), CTL (controls), DIGIT (decimal 0-9), DQUOTE (double quote),
HEXDIG (hexadecimal 0-9/A-F/a-f), LF (line feed), OCTET (any 8-bit
sequence of data), SP (space), VCHAR (any visible USASCII character),
and WSP (whitespace).
1.4.1. Core Rules
The core rules below are defined in Section 1.2.2 of [Part1]:
quoted-string = <quoted-string, defined in [Part1], Section 1.2.2>
token = <token, defined in [Part1], Section 1.2.2>
OWS = <OWS, defined in [Part1], Section 1.2.2>
1.4.2. ABNF Rules defined in other Parts of the Specification
The ABNF rules below are defined in other parts:
field-name = <field-name, defined in [Part1], Section 3.2>
HTTP-date = <HTTP-date, defined in [Part1], Section 6.1>
port = <port, defined in [Part1], Section 2.6>
pseudonym = <pseudonym, defined in [Part1], Section 9.9>
uri-host = <uri-host, defined in [Part1], Section 2.6>
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Internet-Draft HTTP/1.1, Part 6 March 20112. Cache Operation2.1. Response Cacheability
A cache MUST NOT store a response to any request, unless:
o The request method is understood by the cache and defined as being
cacheable, and
o the response status code is understood by the cache, and
o the "no-store" cache directive (see Section 3.2) does not appear
in request or response header fields, and
o the "private" cache response directive (see Section 3.2.2 does not
appear in the response, if the cache is shared, and
o the "Authorization" header field (see Section 4.1 of [Part7]) does
not appear in the request, if the cache is shared, unless the
response explicitly allows it (see Section 2.6), and
o the response either:
* contains an Expires header field (see Section 3.3), or
* contains a max-age response cache directive (see
Section 3.2.2), or
* contains a s-maxage response cache directive and the cache is
shared, or
* contains a Cache Control Extension (see Section 3.2.3) that
allows it to be cached, or
* has a status code that can be served with heuristic freshness
(see Section 2.3.1.1).
In this context, a cache has "understood" a request method or a
response status code if it recognises it and implements any cache-
specific behaviour. In particular, 206 Partial Content responses
cannot be cached by an implementation that does not handle partial
content (see Section 2.1.1).
Note that in normal operation, most caches will not store a response
that has neither a cache validator nor an explicit expiration time,
as such responses are not usually useful to store. However, caches
are not prohibited from storing such responses.
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Internet-Draft HTTP/1.1, Part 6 March 20112.1.1. Storing Partial and Incomplete Responses
A cache that receives an incomplete response (for example, with fewer
bytes of data than specified in a Content-Length header field) can
store the response, but MUST treat it as a partial response [Part5].
Partial responses can be combined as described in Section 4 of
[Part5]; the result might be a full response or might still be
partial. A cache MUST NOT return a partial response to a client
without explicitly marking it as such using the 206 (Partial Content)
status code.
A cache that does not support the Range and Content-Range header
fields MUST NOT store incomplete or partial responses.
2.2. Constructing Responses from Caches
For a presented request, a cache MUST NOT return a stored response,
unless:
o The presented effective request URI (Section 4.3 of [Part1]) and
that of the stored response match, and
o the request method associated with the stored response allows it
to be used for the presented request, and
o selecting header fields nominated by the stored response (if any)
match those presented (see Section 2.7), and
o the presented request and stored response are free from directives
that would prevent its use (see Section 3.2 and Section 3.4), and
o the stored response is either:
* fresh (see Section 2.3), or
* allowed to be served stale (see Section 2.3.3), or
* successfully validated (see Section 2.4).
When a stored response is used to satisfy a request without
validation, a cache MUST include a single Age header field
(Section 3.1) in the response with a value equal to the stored
response's current_age; see Section 2.3.2.
A cache MUST write through requests with methods that are unsafe
(Section 7.1.1 of [Part2]) to the origin server; i.e., a cache must
not generate a reply to such a request before having forwarded the
request and having received a corresponding response.
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Also, note that unsafe requests might invalidate already stored
responses; see Section 2.5.
A cache MUST use the most recent response (as determined by the Date
header field) when more than one suitable response is stored. It can
also forward a request with "Cache-Control: max-age=0" or "Cache-
Control: no-cache" to disambiguate which response to use.
A cache that does not have a clock available MUST NOT used stored
responses without revalidating them on every use. A cache,
especially a shared cache, SHOULD use a mechanism, such as NTP
[RFC1305], to synchronize its clock with a reliable external
standard.
2.3. Freshness Model
When a response is "fresh" in the cache, it can be used to satisfy
subsequent requests without contacting the origin server, thereby
improving efficiency.
The primary mechanism for determining freshness is for an origin
server to provide an explicit expiration time in the future, using
either the Expires header field (Section 3.3) or the max-age response
cache directive (Section 3.2.2). Generally, origin servers will
assign future explicit expiration times to responses in the belief
that the representation is not likely to change in a semantically
significant way before the expiration time is reached.
If an origin server wishes to force a cache to validate every
request, it can assign an explicit expiration time in the past to
indicate that the response is already stale. Compliant caches will
normally validate the cached response before reusing it for
subsequent requests (see Section 2.3.3).
Since origin servers do not always provide explicit expiration times,
a cache MAY assign a heuristic expiration time when an explicit time
is not specified, employing algorithms that use other header field
values (such as the Last-Modified time) to estimate a plausible
expiration time. This specification does not provide specific
algorithms, but does impose worst-case constraints on their results.
The calculation to determine if a response is fresh is:
response_is_fresh = (freshness_lifetime > current_age)
The freshness_lifetime is defined in Section 2.3.1; the current_age
is defined in Section 2.3.2.
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Additionally, clients might need to influence freshness calculation.
They can do this using several request cache directives, with the
effect of either increasing or loosening constraints on freshness.
See Section 3.2.1.
[[ISSUE-no-req-for-directives: there are not requirements directly
applying to cache-request-directives and freshness.]]
Note that freshness applies only to cache operation; it cannot be
used to force a user agent to refresh its display or reload a
resource. See Section 4 for an explanation of the difference between
caches and history mechanisms.
2.3.1. Calculating Freshness Lifetime
A cache can calculate the freshness lifetime (denoted as
freshness_lifetime) of a response by using the first match of:
o If the cache is shared and the s-maxage response cache directive
(Section 3.2.2) is present, use its value, or
o If the max-age response cache directive (Section 3.2.2) is
present, use its value, or
o If the Expires response header field (Section 3.3) is present, use
its value minus the value of the Date response header field, or
o Otherwise, no explicit expiration time is present in the response.
A heuristic freshness lifetime might be applicable; see
Section 2.3.1.1.
Note that this calculation is not vulnerable to clock skew, since all
of the information comes from the origin server.
2.3.1.1. Calculating Heuristic Freshness
If no explicit expiration time is present in a stored response that
has a status code whose definition allows heuristic freshness to be
used (including the following in Section 8 of [Part2]: 200, 203, 206,
300, 301 and 410), a cache MAY calculate a heuristic expiration time.
A cache MUST NOT use heuristics to determine freshness for responses
with status codes that do not explicitly allow it.
When a heuristic is used to calculate freshness lifetime, a cache
SHOULD attach a Warning header field with a 113 warn-code to the
response if its current_age is more than 24 hours and such a warning
is not already present.
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Also, if the response has a Last-Modified header field (Section 6.6
of [Part4]), a cache SHOULD NOT use a heuristic expiration value that
is more than some fraction of the interval since that time. A
typical setting of this fraction might be 10%.
Note: RFC 2616 ([RFC2616], Section 13.9) required that caches do
not calculate heuristic freshness for URIs with query components
(i.e., those containing '?'). In practice, this has not been
widely implemented. Therefore, servers are encouraged to send
explicit directives (e.g., Cache-Control: no-cache) if they wish
to preclude caching.
2.3.2. Calculating Age
HTTP/1.1 uses the Age header field to convey the estimated age of the
response message when obtained from a cache. The Age field value is
the cache's estimate of the amount of time since the response was
generated or validated by the origin server. In essence, the Age
value is the sum of the time that the response has been resident in
each of the caches along the path from the origin server, plus the
amount of time it has been in transit along network paths.
The following data is used for the age calculation:
age_value
The term "age_value" denotes the value of the Age header field
(Section 3.1), in a form appropriate for arithmetic operation; or
0, if not available.
date_value
HTTP/1.1 requires origin servers to send a Date header field, if
possible, with every response, giving the time at which the
response was generated. The term "date_value" denotes the value
of the Date header field, in a form appropriate for arithmetic
operations. See Section 9.3 of [Part1] for the definition of the
Date header field, and for requirements regarding responses
without it.
now
The term "now" means "the current value of the clock at the host
performing the calculation". A cache SHOULD use NTP ([RFC1305])
or some similar protocol to synchronize its clocks to a globally
accurate time standard.
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request_time
The current value of the clock at the host at the time the request
resulting in the stored response was made.
response_time
The current value of the clock at the host at the time the
response was received.
A response's age can be calculated in two entirely independent ways:
1. the "apparent_age": response_time minus date_value, if the local
clock is reasonably well synchronized to the origin server's
clock. If the result is negative, the result is replaced by
zero.
2. the "corrected_age_value", if all of the caches along the
response path implement HTTP/1.1. A cache MUST interpret this
value relative to the time the request was initiated, not the
time that the response was received.
apparent_age = max(0, response_time - date_value);
response_delay = response_time - request_time;
corrected_age_value = age_value + response_delay;
These are combined as
corrected_initial_age = max(apparent_age, corrected_age_value);
The current_age of a stored response can then be calculated by adding
the amount of time (in seconds) since the stored response was last
validated by the origin server to the corrected_initial_age.
resident_time = now - response_time;
current_age = corrected_initial_age + resident_time;
2.3.3. Serving Stale Responses
A "stale" response is one that either has explicit expiry information
or is allowed to have heuristic expiry calculated, but is not fresh
according to the calculations in Section 2.3.
A cache MUST NOT return a stale response if it is prohibited by an
explicit in-protocol directive (e.g., by a "no-store" or "no-cache"
cache directive, a "must-revalidate" cache-response-directive, or an
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Internet-Draft HTTP/1.1, Part 6 March 2011
applicable "s-maxage" or "proxy-revalidate" cache-response-directive;
see Section 3.2.2).
A cache SHOULD NOT return stale responses unless it is disconnected
(i.e., it cannot contact the origin server or otherwise find a
forward path) or otherwise explicitly allowed (e.g., the max-stale
request directive; see Section 3.2.1).
A cache SHOULD append a Warning header field with the 110 warn-code
(see Section 3.6) to stale responses. Likewise, a cache SHOULD add
the 112 warn-code to stale responses if the cache is disconnected.
If a cache receives a first-hand response (either an entire response,
or a 304 (Not Modified) response) that it would normally forward to
the requesting client, and the received response is no longer fresh,
the cache SHOULD forward it to the requesting client without adding a
new Warning (but without removing any existing Warning header
fields). A cache SHOULD NOT attempt to validate a response simply
because that response became stale in transit.
2.4. Validation Model
When a cache has one or more stored responses for a requested URI,
but cannot serve any of them (e.g., because they are not fresh, or
one cannot be selected; see Section 2.7), it can use the conditional
request mechanism [Part4] in the forwarded request to give the origin
server an opportunity to both select a valid stored response to be
used, and to update it. This process is known as "validating" or
"revalidating" the stored response.
When sending such a conditional request, a cache SHOULD add an If-
Modified-Since header field whose value is that of the Last-Modified
header field from the selected (see Section 2.7) stored response, if
available.
Additionally, a cache SHOULD add an If-None-Match header field whose
value is that of the ETag header field(s) from all responses stored
for the requested URI, if present. However, if any of the stored
responses contains only partial content, the cache SHOULD NOT include
its entity-tag in the If-None-Match header field unless the request
is for a range that would be fully satisfied by that stored response.
A 304 (Not Modified) response status code indicates that the stored
response can be updated and reused; see Section 2.8.
A full response (i.e., one with a response body) indicates that none
of the stored responses nominated in the conditional request is
suitable. Instead, a cache SHOULD use the full response to satisfy
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the request and MAY replace the stored response.
If a cache receives a 5xx response while attempting to validate a
response, it MAY either forward this response to the requesting
client, or act as if the server failed to respond. In the latter
case, it MAY return a previously stored response (see Section 2.3.3).
2.5. Request Methods that Invalidate
Because unsafe request methods (Section 7.1.1 of [Part2]) have the
potential for changing state on the origin server, intervening caches
can use them to keep their contents up-to-date.
A cache MUST invalidate the effective Request URI (Section 4.3 of
[Part1]) as well as the URI(s) in the Location and Content-Location
header fields (if present) when the following request methods are
received:
o PUT
o DELETE
o POST
However, a cache MUST NOT invalidate a URI from a Location or
Content-Location header field if the host part of that URI differs
from the host part in the effective request URI (Section 4.3 of
[Part1]). This helps prevent denial of service attacks.
A cache that passes through requests with methods it does not
understand SHOULD invalidate the effective request URI (Section 4.3
of [Part1]).
Here, "invalidate" means that the cache will either remove all stored
responses related to the effective request URI, or will mark these as
"invalid" and in need of a mandatory validation before they can be
returned in response to a subsequent request.
Note that this does not guarantee that all appropriate responses are
invalidated. For example, the request that caused the change at the
origin server might not have gone through the cache where a response
is stored.
2.6. Shared Caching of Authenticated Responses
A shared cache MUST NOT use a cached response to a request with an
Authorization header field (Section 4.1 of [Part7]) to satisfy any
subsequent request unless a cache directive that allows such
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responses to be stored is present in the response.
In this specification, the following Cache-Control response
directives (Section 3.2.2) have such an effect: must-revalidate,
public, s-maxage.
Note that cached responses that contain the "must-revalidate" and/or
"s-maxage" response directives are not allowed to be served stale
(Section 2.3.3) by shared caches. In particular, a response with
either "max-age=0, must-revalidate" or "s-maxage=0" cannot be used to
satisfy a subsequent request without revalidating it on the origin
server.
2.7. Caching Negotiated Responses
When a cache receives a request that can be satisfied by a stored
response that has a Vary header field (Section 3.5), it MUST NOT use
that response unless all of the selecting header fields nominated by
the Vary header field match in both the original request (i.e., that
associated with the stored response), and the presented request.
The selecting header fields from two requests are defined to match if
and only if those in the first request can be transformed to those in
the second request by applying any of the following:
o adding or removing whitespace, where allowed in the header field's
syntax
o combining multiple header fields with the same field name (see
Section 3.2 of [Part1])
o normalizing both header field values in a way that is known to
have identical semantics, according to the header field's
specification (e.g., re-ordering field values when order is not
significant; case-normalization, where values are defined to be
case-insensitive)
If (after any normalization that might take place) a header field is
absent from a request, it can only match another request if it is
also absent there.
A Vary header field-value of "*" always fails to match, and
subsequent requests to that resource can only be properly interpreted
by the origin server.
The stored response with matching selecting header fields is known as
the selected response.
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If no selected response is available, the cache MAY forward the
presented request to the origin server in a conditional request; see
Section 2.4.
2.8. Combining Responses
When a cache receives a 304 (Not Modified) response or a 206 (Partial
Content) response (in this section, the "new" response"), it needs to
create an updated response by combining the stored response with the
new one, so that the updated response can be used to satisfy the
request, and potentially update the cached response.
If the new response contains an ETag, it identifies the stored
response to use. [[TODO-mention-CL: might need language about
Content-Location here]][[TODO-select-for-combine: Shouldn't this be
the selected response?]]
When the new response's status code is 206 (partial content), a cache
MUST NOT combine it with the old response if either response does not
have a validator, and MUST NOT combine it with the old response when
those validators do not match with the strong comparison function
(see Section 4 of [Part4]).
The stored response header fields are used as those of the updated
response, except that
o a cache MUST delete any stored Warning header fields with warn-
code 1xx (see Section 3.6).
o a cache MUST retain any stored Warning header fields with warn-
code 2xx.
o a cache MUST use other header fields provided in the new response
to replace all instances of the corresponding header fields from
the stored response.
A cache MUST use the updated response header fields to replace those
of the stored response (unless the stored response is removed). In
the case of a 206 response, a cache MAY store the combined
representation.
3. Header Field Definitions
This section defines the syntax and semantics of HTTP/1.1 header
fields related to caching.
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Internet-Draft HTTP/1.1, Part 6 March 20113.1. Age
The "Age" header field conveys the sender's estimate of the amount of
time since the response was generated or successfully validated at
the origin server. Age values are calculated as specified in
Section 2.3.2.
Age = "Age" ":" OWS Age-v
Age-v = delta-seconds
Age field-values are non-negative integers, representing time in
seconds.
delta-seconds = 1*DIGIT
If a cache receives a value larger than the largest positive integer
it can represent, or if any of its age calculations overflows, it
MUST transmit an Age header field with a field-value of 2147483648
(2^31). Recipients parsing the Age header field-value SHOULD use an
arithmetic type of at least 31 bits of range.
The presence of an Age header field in a response implies that a
response is not first-hand. However, the converse is not true, since
HTTP/1.0 caches might not implement the Age header field.
3.2. Cache-Control
The "Cache-Control" header field is used to specify directives for
caches along the request/response chain. Such cache directives are
unidirectional in that the presence of a directive in a request does
not imply that the same directive is to be given in the response.
A cache MUST obey the requirements of the Cache-Control directives
defined in this section. See Section 3.2.3 for information about how
Cache-Control directives defined elsewhere are handled.
Note: HTTP/1.0 caches might not implement Cache-Control and might
only implement Pragma: no-cache (see Section 3.4).
A proxy, whether or not it implements a cache, MUST pass cache
directives through in forwarded messages, regardless of their
significance to that application, since the directives might be
applicable to all recipients along the request/response chain. It is
not possible to target a directive to a specific cache.
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Cache-Control = "Cache-Control" ":" OWS Cache-Control-v
Cache-Control-v = 1#cache-directive
cache-directive = cache-request-directive
/ cache-response-directive
cache-extension = token [ "=" ( token / quoted-string ) ]
3.2.1. Request Cache-Control Directives
cache-request-directive =
"no-cache"
/ "no-store"
/ "max-age" "=" delta-seconds
/ "max-stale" [ "=" delta-seconds ]
/ "min-fresh" "=" delta-seconds
/ "no-transform"
/ "only-if-cached"
/ cache-extension
no-cache
The no-cache request directive indicates that a cache MUST NOT use
a stored response to satisfy the request without successful
validation on the origin server.
no-store
The no-store request directive indicates that a cache MUST NOT
store any part of either this request or any response to it. This
directive applies to both private and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a
best-effort attempt to remove the information from volatile
storage as promptly as possible after forwarding it.
This directive is NOT a reliable or sufficient mechanism for
ensuring privacy. In particular, malicious or compromised caches
might not recognize or obey this directive, and communications
networks might be vulnerable to eavesdropping.
Note that if a request containing this directive is satisfied from
a cache, the no-store request directive does not apply to the
already stored response.
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max-age
The max-age request directive indicates that the client is willing
to accept a response whose age is no greater than the specified
time in seconds. Unless the max-stale request directive is also
present, the client is not willing to accept a stale response.
max-stale
The max-stale request directive indicates that the client is
willing to accept a response that has exceeded its expiration
time. If max-stale is assigned a value, then the client is
willing to accept a response that has exceeded its expiration time
by no more than the specified number of seconds. If no value is
assigned to max-stale, then the client is willing to accept a
stale response of any age.
min-fresh
The min-fresh request directive indicates that the client is
willing to accept a response whose freshness lifetime is no less
than its current age plus the specified time in seconds. That is,
the client wants a response that will still be fresh for at least
the specified number of seconds.
no-transform
The no-transform request directive indicates that an intermediary
(whether or not it implements a cache) MUST NOT change the
Content-Encoding, Content-Range or Content-Type request header
fields, nor the request representation.
only-if-cached
The only-if-cached request directive indicates that the client
only wishes to return a stored response. If it receives this
directive, a cache SHOULD either respond using a stored response
that is consistent with the other constraints of the request, or
respond with a 504 (Gateway Timeout) status code. If a group of
caches is being operated as a unified system with good internal
connectivity, a member cache MAY forward such a request within
that group of caches.
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cache-response-directive =
"public"
/ "private" [ "=" DQUOTE 1#field-name DQUOTE ]
/ "no-cache" [ "=" DQUOTE 1#field-name DQUOTE ]
/ "no-store"
/ "no-transform"
/ "must-revalidate"
/ "proxy-revalidate"
/ "max-age" "=" delta-seconds
/ "s-maxage" "=" delta-seconds
/ cache-extension
public
The public response directive indicates that a response whose
associated request contains an 'Authentication' header MAY be
stored (see Section 2.6).
private
The private response directive indicates that the response message
is intended for a single user and MUST NOT be stored by a shared
cache. A private cache MAY store the response.
If the private response directive specifies one or more field-
names, this requirement is limited to the field-values associated
with the listed response header fields. That is, a shared cache
MUST NOT store the specified field-names(s), whereas it MAY store
the remainder of the response message.
Note: This usage of the word private only controls where the
response can be stored; it cannot ensure the privacy of the
message content. Also, private response directives with field-
names are often handled by implementations as if an unqualified
private directive was received; i.e., the special handling for the
qualified form is not widely implemented.
no-cache
The no-cache response directive indicates that the response MUST
NOT be used to satisfy a subsequent request without successful
validation on the origin server. This allows an origin server to
prevent a cache from using it to satisfy a request without
contacting it, even by caches that have been configured to return
stale responses.
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If the no-cache response directive specifies one or more field-
names, this requirement is limited to the field-values associated
with the listed response header fields. That is, a cache MUST NOT
send the specified field-name(s) in the response to a subsequent
request without successful validation on the origin server. This
allows an origin server to prevent the re-use of certain header
fields in a response, while still allowing caching of the rest of
the response.
Note: Most HTTP/1.0 caches will not recognize or obey this
directive. Also, no-cache response directives with field-names
are often handled by implementations as if an unqualified no-cache
directive was received; i.e., the special handling for the
qualified form is not widely implemented.
no-store
The no-store response directive indicates that a cache MUST NOT
store any part of either the immediate request or response. This
directive applies to both private and shared caches. "MUST NOT
store" in this context means that the cache MUST NOT intentionally
store the information in non-volatile storage, and MUST make a
best-effort attempt to remove the information from volatile
storage as promptly as possible after forwarding it.
This directive is NOT a reliable or sufficient mechanism for
ensuring privacy. In particular, malicious or compromised caches
might not recognize or obey this directive, and communications
networks might be vulnerable to eavesdropping.
must-revalidate
The must-revalidate response directive indicates that once it has
become stale, a cache MUST NOT use the response to satisfy
subsequent requests without successful validation on the origin
server.
The must-revalidate directive is necessary to support reliable
operation for certain protocol features. In all circumstances a
cache MUST obey the must-revalidate directive; in particular, if a
cache cannot reach the origin server for any reason, it MUST
generate a 504 (Gateway Timeout) response.
A server SHOULD send the must-revalidate directive if and only if
failure to validate a request on the representation could result
in incorrect operation, such as a silently unexecuted financial
transaction.
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proxy-revalidate
The proxy-revalidate response directive has the same meaning as
the must-revalidate response directive, except that it does not
apply to private caches.
max-age
The max-age response directive indicates that response is to be
considered stale after its age is greater than the specified
number of seconds.
s-maxage
The s-maxage response directive indicates that, in shared caches,
the maximum age specified by this directive overrides the maximum
age specified by either the max-age directive or the Expires
header field. The s-maxage directive also implies the semantics
of the proxy-revalidate response directive.
no-transform
The no-transform response directive indicates that an intermediary
(regardless of whether it implements a cache) MUST NOT change the
Content-Encoding, Content-Range or Content-Type response header
fields, nor the response representation.
3.2.3. Cache Control Extensions
The Cache-Control header field can be extended through the use of one
or more cache-extension tokens, each with an optional value.
Informational extensions (those that do not require a change in cache
behavior) can be added without changing the semantics of other
directives. Behavioral extensions are designed to work by acting as
modifiers to the existing base of cache directives. Both the new
directive and the standard directive are supplied, such that
applications that do not understand the new directive will default to
the behavior specified by the standard directive, and those that
understand the new directive will recognize it as modifying the
requirements associated with the standard directive. In this way,
extensions to the cache-control directives can be made without
requiring changes to the base protocol.
This extension mechanism depends on an HTTP cache obeying all of the
cache-control directives defined for its native HTTP-version, obeying
certain extensions, and ignoring all directives that it does not
understand.
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For example, consider a hypothetical new response directive called
"community" that acts as a modifier to the private directive. We
define this new directive to mean that, in addition to any private
cache, any cache that is shared only by members of the community
named within its value may cache the response. An origin server
wishing to allow the UCI community to use an otherwise private
response in their shared cache(s) could do so by including
Cache-Control: private, community="UCI"
A cache seeing this header field will act correctly even if the cache
does not understand the community cache-extension, since it will also
see and understand the private directive and thus default to the safe
behavior.
A cache MUST be ignore unrecognized cache directives; it is assumed
that any cache directive likely to be unrecognized by an HTTP/1.1
cache will be combined with standard directives (or the response's
default cacheability) such that the cache behavior will remain
minimally correct even if the cache does not understand the
extension(s).
The HTTP Cache Directive Registry defines the name space for the
cache directives.
A registration MUST include the following fields:
o Cache Directive Name
o Pointer to specification text
Values to be added to this name space are subject to IETF review
([RFC5226], Section 4.1).
The registry itself is maintained at
<http://www.iana.org/assignments/http-cache-directives>.
3.3. Expires
The "Expires" header field gives the date/time after which the
response is considered stale. See Section 2.3 for further discussion
of the freshness model.
The presence of an Expires field does not imply that the original
resource will change or cease to exist at, before, or after that
time.
The field-value is an absolute date and time as defined by HTTP-date
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in Section 6.1 of [Part1]; a sender MUST use the rfc1123-date format.
Expires = "Expires" ":" OWS Expires-v
Expires-v = HTTP-date
For example
Expires: Thu, 01 Dec 1994 16:00:00 GMT
Note: If a response includes a Cache-Control field with the max-
age directive (see Section 3.2.2), that directive overrides the
Expires field. Likewise, the s-maxage directive overrides Expires
in shared caches.
A server SHOULD NOT send Expires dates more than one year in the
future.
A cache MUST treat other invalid date formats, especially including
the value "0", as in the past (i.e., "already expired").
3.4. Pragma
The "Pragma" header field is used to include implementation-specific
directives that might apply to any recipient along the request/
response chain. All pragma directives specify optional behavior from
the viewpoint of the protocol; however, some systems MAY require that
behavior be consistent with the directives.
Pragma = "Pragma" ":" OWS Pragma-v
Pragma-v = 1#pragma-directive
pragma-directive = "no-cache" / extension-pragma
extension-pragma = token [ "=" ( token / quoted-string ) ]
When the no-cache directive is present in a request message, a cache
SHOULD forward the request toward the origin server even if it has a
stored copy of what is being requested. This pragma directive has
the same semantics as the no-cache response directive (see
Section 3.2.2) and is defined here for backward compatibility with
HTTP/1.0. A client SHOULD include both header fields when a no-cache
request is sent to a server not known to be HTTP/1.1 compliant. A
cache SHOULD treat "Pragma: no-cache" as if the client had sent
"Cache-Control: no-cache".
Note: Because the meaning of "Pragma: no-cache" as a header field
is not actually specified, it does not provide a reliable
replacement for "Cache-Control: no-cache" in a response.
This mechanism is deprecated; no new Pragma directives will be
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defined in HTTP.
3.5. Vary
The "Vary" header field conveys the set of header fields that were
used to select the representation.
Caches use this information, in part, to determine whether a stored
response can be used to satisfy a given request; see Section 2.7.
determines, while the response is fresh, whether a cache is permitted
to use the response to reply to a subsequent request without
validation; see Section 2.7.
In uncacheable or stale responses, the Vary field value advises the
user agent about the criteria that were used to select the
representation.
Vary = "Vary" ":" OWS Vary-v
Vary-v = "*" / 1#field-name
The set of header fields named by the Vary field value is known as
the selecting header fields.
A server SHOULD include a Vary header field with any cacheable
response that is subject to server-driven negotiation. Doing so
allows a cache to properly interpret future requests on that resource
and informs the user agent about the presence of negotiation on that
resource. A server MAY include a Vary header field with a non-
cacheable response that is subject to server-driven negotiation,
since this might provide the user agent with useful information about
the dimensions over which the response varies at the time of the
response.
A Vary field value of "*" signals that unspecified parameters not
limited to the header fields (e.g., the network address of the
client), play a role in the selection of the response representation;
therefore, a cache cannot determine whether this response is
appropriate. A proxy MUST NOT generate the "*" value.
The field-names given are not limited to the set of standard header
fields defined by this specification. Field names are case-
insensitive.
3.6. Warning
The "Warning" header field is used to carry additional information
about the status or transformation of a message that might not be
reflected in the message. This information is typically used to warn
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about possible incorrectness introduced by caching operations or
transformations applied to the payload of the message.
Warnings can be used for other purposes, both cache-related and
otherwise. The use of a warning, rather than an error status code,
distinguishes these responses from true failures.
Warning header fields can in general be applied to any message,
however some warn-codes are specific to caches and can only be
applied to response messages.
Warning = "Warning" ":" OWS Warning-v
Warning-v = 1#warning-value
warning-value = warn-code SP warn-agent SP warn-text
[SP warn-date]
warn-code = 3DIGIT
warn-agent = ( uri-host [ ":" port ] ) / pseudonym
; the name or pseudonym of the server adding
; the Warning header field, for use in debugging
warn-text = quoted-string
warn-date = DQUOTE HTTP-date DQUOTE
Multiple warnings can be attached to a response (either by the origin
server or by a cache), including multiple warnings with the same code
number, only differing in warn-text.
When this occurs, the user agent SHOULD inform the user of as many of
them as possible, in the order that they appear in the response.
Systems that generate multiple Warning header fields SHOULD order
them with this user agent behavior in mind. New Warning header
fields SHOULD be added after any existing Warning headers fields.
Warnings are assigned three digit warn-codes. The first digit
indicates whether the Warning is required to be deleted from a stored
response after validation:
o 1xx Warnings describe the freshness or validation status of the
response, and so MUST be deleted by a cache after validation.
They can only be generated by a cache when validating a cached
entry, and MUST NOT be generated in any other situation.
o 2xx Warnings describe some aspect of the representation that is
not rectified by a validation (for example, a lossy compression of
the representation) and MUST NOT be deleted by a cache after
validation, unless a full response is returned, in which case they
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MUST be.
If an implementation sends a message with one or more Warning header
fields to a receiver whose version is HTTP/1.0 or lower, then the
sender MUST include in each warning-value a warn-date that matches
the Date header field in the message.
If a system receives a message with a warning-value that includes a
warn-date, and that warn-date is different from the Date value in the
response, then that warning-value MUST be deleted from the message
before storing, forwarding, or using it. (preventing the consequences
of naive caching of Warning header fields.) If all of the warning-
values are deleted for this reason, the Warning header field MUST be
deleted as well.
The following warn-codes are defined by this specification, each with
a recommended warn-text in English, and a description of its meaning.
110 Response is stale
A cache SHOULD include this whenever the returned response is
stale.
111 Revalidation failed
A cache SHOULD include this when returning a stale response
because an attempt to validate the response failed, due to an
inability to reach the server.
112 Disconnected operation
A cache SHOULD b include this if it is intentionally disconnected
from the rest of the network for a period of time.
113 Heuristic expiration
A cache SHOULD include this if it heuristically chose a freshness
lifetime greater than 24 hours and the response's age is greater
than 24 hours.
199 Miscellaneous warning
The warning text can include arbitrary information to be presented
to a human user, or logged. A system receiving this warning MUST
NOT take any automated action, besides presenting the warning to
the user.
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214 Transformation applied
MUST be added by a proxy if it applies any transformation to the
representation, such as changing the content-coding, media-type,
or modifying the representation data, unless this Warning code
already appears in the response.
299 Miscellaneous persistent warning
The warning text can include arbitrary information to be presented
to a human user, or logged. A system receiving this warning MUST
NOT take any automated action.
4. History Lists
User agents often have history mechanisms, such as "Back" buttons and
history lists, that can be used to redisplay a representation
retrieved earlier in a session.
The freshness model (Section 2.3) does not necessarily apply to
history mechanisms. I.e., a history mechanism can display a previous
representation even if it has expired.
This does not prohibit the history mechanism from telling the user
that a view might be stale, or from honoring cache directives (e.g.,
Cache-Control: no-store).
5. IANA Considerations5.1. Cache Directive Registry
The registration procedure for HTTP Cache Directives is defined by
Section 3.2.3 of this document.
The HTTP Cache Directive Registry shall be created at
<http://www.iana.org/assignments/http-cache-directives> and be
populated with the registrations below:
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